Linear aromatic condensation polyimide film is a material of choice for use in many aerospace as well as commercial applications where long term durability at temperatures in the range of 200.degree.-300.degree. C. are needed and where shorter times at temperatures exceeding 300.degree. C. are necessary. Because of their inherent toughness and flexibility, low density, remarkable thermal stability, radiation resistance and mechanical strength, aromatic polyimide films have excellent potential for use as materials on large space structures.
The need exists for high temperature, flexible polymeric film and coating materials that have high optical transparency in the the 300-600 nm range of the electromagnetic spectrum for applications on such space components as antennae, solar cells and thermal control coating systems. Although there are available several classes of polymers which are transparent/colorless such as polyesters or aliphatic polyimides, these materials have limited long term thermal stability. A major drawback for using the thermally stable aromatic polyimides for the above stated space applications has been their poor transparency in the visible range of the electromagnetic spectrum. Traditionally, linear all-aromatic condensation polyimides are known for their bright yellow color. Commercial aromatic polyimide film has been evaluated for applications on space solar cells and thermal control coating systems but was found to be only about 70% transparent (depending on thickness) at the solar wavelength of interest (500 nm). A further drawback for using the intensely colored aromatic polyimides as space films or coatings is that although they are approximately 70% transparent, upon aging in a space environment they become even less transparent (as low as 30% transparent).
By the present invention, aromatic condensation polyimide films are produced which are essentially colorless to totally colorless. These materials provide 90% transparency at the visible wavelength of 500 nm compared to 60-70% transparency for commercial polyimide film of the same thickness. It is anticipated that these optically transparent/achromatic films will prove highly useful as film and coating materials for aerospace applications where high transparency and thermal stability are necessary criteria. This improvement in transparency has been made without sacrificing other advantageous polymer properties.
Accordingly, an object of the present invention is to provide a process for increasing the optical transparency of linear aromatic condensation polyimide film.
Another object of the present invention is to provide a process for preparing aromatic polyimides for use as films and/or coatings having improved optical transparency at wavelengths in the visible region of the electromagnetic spectrum useful for transmitting solar energy as needed in such applications as solar cell covers or second surface mirror coatings in thermal control coating systems.
A further object of the present invention is an aromatic polyimide having improved optical transparency.